This invention relates to optical fiber communication systems and, more particularly, to arrangements for tapping signal power from an optical fiber waveguide without requiring that the fiber be terminated or broken.
Rapid progress has been made in the past few years in the design and fabrication of optical fiber waveguiding structures. There are now available several different fiber structures which are capable of transmitting large quantities of information via modulated optical waves or pulses with transmission losses as low as two decibels per kilometer. It is expected that some day such fibers will replace, at least in part, the wire pairs, coaxial cables and metallic waveguides now used in conventional communication systems. The advantages of fiber systems over conventional systems include the small physical size and light weight of the fiber waveguides, the broad bandwidth capabilities which afford flexibility in the selection of a bandwidth to be utilized in any given system, the nonconductive, noninductive properties of the fiber waveguides, and the potentially low cost of fiber materials and fabrication. The prospects of future use of the fiber systems are indeed wide-ranging, and continue to expand.
The earliest implementation of fiber systems is likely to involve multi-terminal information transfer over short distance optical fiber links using light-emitting diodes, which have now been developed to the point of having sufficiently long operating lives, as the signal source. Because of the light weight and the immunity to electromagnetic interference of fiber systems, fiber optical data bus links have been proposed for the transmission of control and intercom signals on board aircraft and ships. Other potential applications include interoffice trunks, such as those interconnecting telephone central offices within a city, "on-premise" distribution links within a building or between adjacent buildings, and data bus links in computer or industrial control systems.
In the longer range future, optical fiber systems are likely to be used for the high capacity transmission of digital information over long distance fiber links, with lasers as the signal source. Intercity telecommunication links may thus some day be provided using optical fibers. It appears likely that repeater spacings of several kilometers or more and information transmission rates in the gigabit range will become technically feasible with such systems.
Whatever the application, it is clear that arrangements will be required for extracting signal wave information from the optical fiber waveguides. To monitor and control the transmission through a fiber link, for example, it may be required to sample the signal propagating through the individual fiber waveguides periodically along the link. Optical data bus links will require that signals be extracted for utilization at numerous selected points along the link. In most instances, it would be desirable if a portion of the signal propagating through the fiber could be tapped therefrom without breaking or terminating the fiber. Fiber terminations can add unwanted optical losses to the system, and would unfavorably increase the need for highly precise fiber splicing and interconnecting arrangements.